Are Markups Really SO Bad?
Concentration is a terrible measure of [insert basically anything people actually care about]. Have I said that before? Concentration tells us nothing about market power, efficiency, or whether policy changes can do anything to increase welfare. Economists know that, especially industrial organization (IO) economists.
If we want to measure market power for a seller, a better measure is the markup, defined as the ratio of price over marginal cost. If we want to measure market power for a buyer, we can look at the markdown. Either one is a better measure of market power (possibly bad) and often the very definition of market power.